1. Field of the Invention
The present invention relates to an austenitic steel having improved creep strength.
2. Description of the prior art
Recent developments in the formulation of austenitic steel alloys have produced austenitic steels having desired properties such as high temperature oxidation resistance, good cold workability, weldability and high mechanical strength at ambient temperature. Research continues, however, into providing a steel alloy having improved creep strength, which is useful for steel annealing box covers which operate at temperatures around 800.degree. C.
Recently, Avesta has developed a new alloy grade designated Avesta 253MA.TM. which provides improved creep strength over its prior steel alloys. This development is discussed in U.S. Pat. No. 4,224,062. Therein, an austenitic steel alloy having improved high temperature creep strength is formed by incorporating a rare earth metal, such as lanthanum and the other lanthanides, and an alkaline earth metal, such as the group 2a elements calcium, strontium and barium, into a fully austenitic steel. In a preferred embodiment, calcium in the amount 0.002-0,006 % by weight is used as the alkaline earth metal and cerium in the amount 0.03-0.07 % by weight is used as the rare earth metal. Even with the improved creep strength afforded by the alloy disclosed in U.S. Pat. No. 4,224,062, alloy 253MA.TM. provides only a marginal improvement in creep strength over existing steel alloys.
Table I below sets forth the expected average creep strain at 700.degree. C. for 253MA.TM. steel alloy and 309 steel alloy, an existing austenitic steel alloy recognized as needing improved creep performance. As can be seen, even with the addition of the lanthanide rare earth metals and alkaline earth metals, the increased creep strain performance of 253MA.TM. steel alloy is minimal.
TABLE I ______________________________________ Creep Strain At 700.degree. C. (MPa) 253 MA .TM. 309 ______________________________________ 1,000 hours 74 70 10,000 hours 44 40 ______________________________________
Although the addition of a lanthanide rare earth metal performs satisfactorily in the 253MA.TM. alloy, the addition of a lanthanide metal lessens the weldability of certain alloy compositions. Notably, the addition of a rare earth lanthanide metal to alloy 309 results in an alloy having lessened weldability performance. Thus, there is a need for an alloy having improved creep strength which does not rely on the addition of a rare earth metal to provide that improved property.
It is also desired in a steel alloy to have improved carburization resistance. The typical approach to improve carburization resistance is to increase the amount of silicon in the steel alloy. However, the addition of silicon to most austenitic steel alloys reduces the creep strength of the alloys and worsens fusion cracking in the weldments in the alloys. Consequently, there is a need for a steel alloy having improved carburization resistance which does not rely on the addition of higher silicon content in the alloy composition.